Wettability is the ability of a liquid to spread out and leave a trace on a solid. This characteristic of liquids is related to the molecular interactions between the liquid molecules and the molecules of the surface.
The interaction between the molecules of the liquid and the surface, determines the contact angle θ, which is the angle formed between the exposed surface of the liquid and the contact surface with the solid.
When the contact angle is small (θ<90°) indicates that the wettability is high, and the fluid will spread over the surface; these surfaces are called hydrophilic. A large angle of contact (θ>90°) means a decrease in the fluid contact surface; in these case the wettability is low and the liquid forms a compact drop; these surfaces is called hydrophobic.
For a liquid such as water, a glass surface behaves as a hydrophilic surface, since the angle θ = 30°. So, it is said that water can wet a glass surface. A brass surface behaves as hydrophobic for water, since the contact angle is greater than 90°; in this case water can not wet the surface of brass.